1. Field of the Invention
This invention relates to rechargeable cells and more particularly to rechargeable cells of the type which include an oxygen sensing electrode.
2. Description of Prior Art
The prior art includes a number of examples of sealed rechargeable nickel-cadmium cells which include sensing electrodes for sensing the accumulation of oxygen within the cell during charging and even for reducing the amount of charge when the oxygen pressure is indicated to have reached a predetermined level. An example of such a sensing electrode is shown in U.S. Pat. No. 3,554,804, Carson, Jr., which is assigned to the assignee of the present invention.
The prior art discloses the use of silver on the negative electrode of a rechargeable nickel-cadmium cell for enhancing the recombination of oxygen and thereby reducing the pressure build-up of oxygen during charging. An example of this prior art is U.S. Pat. No. 3,877,985, Rampel, also assigned to the assignee of the present invention. In the particular structure disclosed in that patent the negative electrode comprises a substrate with nickel particles bonded thereon, an active material, namely cadmium, impregnated in the pores of the nickel and a small deposit of silver particles dispersed in intimate contact with the cadmium.
Some prior art oxygen sensing electrodes have been formed on a substrate of nickel or nickel-plated steel coated with sintered nickel powder. A sheet of hydrophobic fluorocarbon polymer, particularly polytetrafluoroethylene, has been applied to one side of the electrode as disclosed, for example, in the aforementioned U.S. Pat. No. 3,554,804, Carson, Jr. This construction allowed oxygen forming in the cell to diffuse through the polytetrafluoroethylene film, which is previous to oxygen, to produce a voltage representative of the oxygen forming in the cell. This voltage, appearing across an external resistor between the sensing electrode and the negative electrode, could be employed simply to indicate the oxygen pressure in the cell so that action could be taken to discontinue or reduce the charging rate when the oxygen pressure is indicated to have reached a predetermined level. Alternatively, the sensed voltage could be used, for example, as shown in the aformentioned U.S. Pat. No. 3,554,804, to actuate a control for automatically reducing the charging current supplied from the charging source. However, while a polytetrafluoroethylene layer in film form, as in the Carson, Jr. sensing electrode, is pervious to oxygen, and hence allows the oxygen to pass to the electrode, it is impervious to ionic current flow. Therefore, prior art sensing electrodes of this type could not be used between negative and positive electrodes without a sacrifice in cell performance.
Rechargeable electrodes have been disclosed in the prior art (see U.S. Pat. No. 3,630,781, Rampel), which utilized polytetrafluoroethylene in dispersion form. In this form the polytetrafluoroethylene is pervious to ionic current as well as oxygen. No reference is made in this patent to the use of polytetrafluoroethylene in this form on an oxygen sensing electrode.
Silver has been used on a sensing electrode in the prior art, in fuel cells, for example since it is known that silver is more catalytically active to oxygen than, for example, nickel. However, attempts to use it on a sensing electrode in nickel-cadmium cells employing an alkaline electrolyte, such as potassium hydroxide, have not been successful because of the solubility of silver oxide (formed by combination of the silver with the oxygen generated during charging) in the alkaline electrolyte and the consequent migration of the silver oxide throughout the cell and consequent loss of silver from the sensing electrode and also damage to the cell.
In accordance with the present invention these limitations of the prior art are overcome and an oxygen sensing electrode is provided which develops a signal of greater magnitude across a lower load resistance than that of prior art sensing electrodes for a given oxygen pressure developed in the cell. The sensing electrode of this invention is pervious to oxygen, so that oxygen developed at the positive electrode passes readily to the sensing electrode for developing a signal voltage indicative of the oxygen pressure, and is also permeable to ionic current so that the sensing electrode may be placed within the cell between positive and negative electrodes without any adverse effect on the performance of the cell. Moreover, because of the employment of silver which is more catalytically active to oxygen, a greater external signal voltage is developed between the sensing electrode and the negative electrode even with a substantially smaller external resistor. Finally, despite the utilization of silver in an alkaline electrolyte, the construction of the sensing electrode of this invention is such that no significant migration of silver oxide occurs.
It is, therefore, an object of this invention to provide an improved rechargeable cell including an oxygen sensing electrode which provides a greater signal voltage across a lower load resistance for a given oxygen pressure in the cell.
It is another object of this invention to provide a sensing electrode for such a cell which may be employed between positive and negative electrodes thereof.
It is a further object of this invention to provide a sensing electrode, using silver as a component thereof, which avoids migration of silver oxide through the cell.